Studies of bone strength in vitro have demonstrated that the decrease in bone strength in both the spine and femur is directly proportional to the mineral content. For this reason, bone densitometry has been used extensively for the determination of bone loss in clinical diagnosis and monitoring. A variety of methods have been used, such as single and dual photon absorptiometry (SPA and DPA, respectively) and quantitative computer tomography (QCT). However, these methods are time-consuming, dependent upon the availability of sophisticated and expensive equipment, and thus expensive and ill-suited for widespread implementation. Thus, there has been a long-felt need in the prior art for a simple, efficient, and low cost methodology for measuring bone density as it is an effective diagnosis for the onset of osteoporosis, a debilitating disease commonly found in post-pregnancy and post-menopausal women. Treatment of osteoporosis is most effective if the disease is detected early whereupon hormonal treatment may be commenced. However, because of the increased risk of side effects, hormonal treatment is not desired to be begun until such time as the disease has been detected. Additionally, continuous measurement of bone density over time may readily disclose the effectiveness of treatment, leading to adjustments in the treatment protocols balanced against the attendant side effect risk.
Still another medical situation in which bone integrity is important is the healing process of bone fractures. There is a phenomenon known as non-union healing in which a bone fracture fails to knit properly to thereby return the bone to its pre-fracture integrity. Obviously, x-rays may be used to follow the healing process, but this methodology is expensive and undesirable in that it repeatedly exposes a body part to x-rays. The inventor herein has previously participated in studies which noted the relationship of the vibrational response, and specifically the determination of the natural frequency shift and phase angle shift as being related, indirectly, to the progress of fracture healing. This previous experimental work utilized cadaver bones and its application to living patients is limited for the reasons as discussed in his previously published article. See Monitoring of Fracture Healing Bilateral and Axial Vibration Analysis, Journal of Biomechanics, Vol. 23, No. 4, 1990.
In order to solve these and other problems in the prior art, the inventor herein has succeeded in developing a method and apparatus for determining bone integrity by measuring its vibrational response to a stimulus and determining therefrom its modal damping factor (hereinafter damping factor).
In general, the inventor has developed two techniques for measuring the damping factor of any bone or other body hard tissue. Both techniques include the basic method of coupling a transducer to the bone, the transducer having the capability of measuring the vibrational response and producing an electrical output, and then from that electrical output determining the damping factor through a programmed electronic logic device, such as a computer. In a first implementation of this method, an impulse of energy may be applied to the bone, such as by striking it, in order to generate a vibration in the bone at its natural frequency. This vibration, and its decreasing amplitude, may then be measured and used to calculate the damping factor. In a second implementation of this same method, a continuous input of energy can be applied to the bone, such as by driving a speaker with a frequency generator and coupling the speaker to the bone, such that a continuous vibrational input may be provided at about the natural frequency of the bone. Ideally, the frequency generator is adjustable so that it may be tuned to this natural frequency. The same transducer and computer may then be used to calculate the damping factor through a different mathematical analysis which depends upon the half power bandwidth of the response and its center frequency.
In proving the efficacy of the methodology disclosed and claimed herein, the inventor has conducted several experiments on bones. In doing so, the inventor has discovered that the change in the mechanical properties, such as the damping factor, is one order of magnitude greater than the corresponding change in bone density. Thus, measurement of the damping factor is sensitive to and useful in determining bone density. Furthermore, while it is desired to locate the pick-up transducer close to the bone in order to increase the measured output and amplitude of the vibrational response to thereby minimize measurement errors, it has been found that the flesh which surrounds the bone has minimal effect in the measurement as the bone dominates the damping factor effect at the lower natural frequencies not the flesh. Furthermore, as damping is a measure of the loss of strain energy during one vibratory cycle in the stress bearing part of the bone, it is relatively insensitive to the method of support for the bone. On the other hand, other mechanical properties, such as the natural frequency utilized in the inventor's prior published article depend very much on the method of support and the surrounding flesh, and are not nearly as sensitive, which thus makes those mechanical properties impractical to use in measuring and monitoring bone density levels and changes thereto over time.
The inventor's approach of using the damping factor as an indicator of bone density is also intellectually satisfying in that there is a rationale for the experimentally measured variations in the damping factor. It is generally understood and believed that loss of mass, or decrease in density, of a bone is due to the loss of mineral and a resultant void nucleation that, in turn, results in stress concentration and premature facture. This void nucleation is detected by a change in the damping factor as the bone becomes more porous and "more able" to dampen vibrations induced therein. Thus, the measurement of the damping factor is seen to be a direct measurement of this void nucleation and, hence, a direct indication of the "integrity" of the bone.
The inventor's techniques may be readily applied to the diagnosis and treatment of osteoporosis. In the first instance, the density of a particular bone of a patient may be determined by measuring its damping factor, and this particular bone's density may be determined in the same manner over time as the patient is treated, and these bone densities compared to thereby detect any changes in bone density. Obviously, a decrease in bone density in medically significant amounts would be indicative of the onset of osteoporosis. Alternately, it would be hoped that treatment, perhaps through exercise, would be helpful in increasing or at least forestalling the decrease in bone density. Thus, these techniques may be useful in measuring the effectiveness of treatments so that treatment protocols could be altered as the patient is treated over time. This methodology and use of the damping factor depends upon a relative comparison of damping factor measurements for the same bone in the same patient over time. An alternate methodology would take advantage of standardized bone density values, yet to be determined, for patients of a particular age, sex, fitness level, particular bone, etc. against which a particular patient's bone density measurement may be compared in order to determine their potential for having osteoporosis. As the inventor has recently developed the present invention, there has not been an opportunity to determine these standardized values. However, it is believed to be a straightforward matter for one of ordinary skill in the art to utilize the present invention and measure a statistically significant group of individuals in order to determine these standardized values and the particular factors important in differentiating members of the group.
While the principal advantages and features of the present invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and description of the preferred embodiment which follow.